A thin film transistor liquid crystal display (TFT-LCD) has many merits such as small volume, lower power consumption, and being free of radiation and is gaining a important role in the flat panel display market. As for a TFT-LCD, product quality, yield and price are mainly determined by the structure and the manufacturing process of an array substrate structure thereof.
In order to effectively decrease price and increase yield of a TFT-LCD, the process of manufacturing the array substrate (active matrix driven TFT array) of a TFT-LCD has been simplified gradually. The method has been developed from the initial seven-mask process to the current four-mask process that is based on slit photolithography technology. The key points of the four-mask process is to use a slit photolithography process to replace both the second photolithography (active layer photolithography) process and the third photolithography (source/drain layer photolithography) process in a traditional five-mask process. The four-mask process is described as follows: first forming a gate electrode with a first photolithography process; depositing a gate insulating layer, a semiconductor layer, a doped semiconductor layer (ohmic contact layer) and a source/drain metal layer on the gate electrode sequentially; performing the second photolithography process with a slit mask, and forming patterns of a data line, an active layer, source/drain electrodes and a TFT channel by a wet etching and a multi-step etching (etching the semiconductor layer→ashing photoresist pattern→dry etching→etching the doped semiconductor layer); depositing a passivation layer, and forming a via hole in the passivation layer with a third photolithography process; finally, depositing a transparent conductive layer, and forming a pixel electrode with a fourth photolithography process. It is necessary for the second photolithography process to form the patterns of the active layer, the source/drain electrodes and the TFT channel by means of the slit lithography technique. The principle of the slit lithography technique is to form slits of a size on a mask and control the transmission of the light for exposing with light diffraction due to the presences of slits, so as to selectively control the thickness of the exposed and developed photoresist. Further, the thickness of the photoresist directly determines the aspect ration of the TFT device, i.e., the electrical characteristic of the TFT device.
Practice shows that the four-mask process embodies two disadvantages. Due to a limitation on the accuracy of manufacturing a mask with slits and the influence of the distribution uniformity of the transmission of the mask and the uniformity of the subsequent photoresist ashing process, there exists some difference for the transmission in different areas, and this difference may in turn cause the various defects in the TFT channel, decreasing the electrical characteristics uniformity of the TFT across the whole substrate and degrading the display quality of the TFT-LCD. As one of the key processes of the slit photolithography process, the multi-step etching process is used to form the patterns of the active layer and the source/drain together with the TFT channel and comprises etching of the semiconductor layer, ashing of the photoresist at the channel, and etching of the source/drain electrodes at the channel and etching of the doped semiconductor layer. These etching processes are performed in the same apparatus sequentially, which tends to cause the various kinds of defects in pixels because of the slit photolithography process, so that the yield and the product quality are difficult to be ensured.